Conventionally, a communication apparatus for wireless communication between terminals has adopted the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) scheme as an access scheme, in order that the terminal can achieve wireless communication in an autonomous distributed manner.
The CSMA/CA scheme is generally known as a method that avoids wireless communication collision, in which when a terminal wirelessly transmits information, the terminal waits for a random back-off time before transmission and then performs carrier sensing performed for confirming whether or not a peripheral terminal is in communication, and while the peripheral terminal is in transmission, the own terminal further waits for a random back-off time before transmission and if the peripheral terminal is not in transmission, the own terminal starts transmission.
On the other hand, studies have been made for a method of putting a driving support system into practical use by utilizing a communication apparatus that performs road-to-vehicle communication for communication between a fixed terminal such as a roadside device and a mobile terminal or vehicle-to-vehicle communication for communication between mobile terminals. In this case, an application of the communication apparatus generally adopts such a communication method that terminal information such as the position of each terminal, the moving speed, and the vehicle type are transmitted between terminals in each certain period.
In the CSMA/CA scheme, in a case of periodically transmitting the terminal information, if the number of communication terminals within a communication range increases, the reliability of communication would be deteriorated to make it impossible to provide the driving support system, because: each terminal repeatedly waits for a back-off time before transmission so that congestion occurs that makes transmission impossible; transmission timings after the back-off time coincide with each other with a higher probability so that wireless communication packets collide with each other; collision occurs with a wireless communication packet of a hidden terminal, that is, a terminal that cannot be found by carrier sensing.
Moreover, even though congestion or packet collision occurs, the application that periodically transmits the terminal information cannot detect the occurrence of congestion or collision, and therefore in a next period, transmission is requested at the same timing and in the same period by the CSMA/CA. Thus, the possibility that information failing to be transmitted will be accumulated or the possibility that transmitted packets will collide with each other is still not reduced.
Therefore, Patent Document 1 discloses a wireless communication method in which, in the CSMA/CA scheme, a time when wireless transmission is performed is stored as a base time, and a time after elapse of a time interval corresponding to one period since the base time is set as a transmission timing in the next period, to thereby reduce the possibility that packet collision will occur in every period.
Patent Document 2 discloses a communication control apparatus in which, in the CSMA/CA scheme, a time after elapse of one period since a previous transmission timing is set as a base time, and a timing of periodically performing wireless communication is shifted back and forth by a predetermined time period in each period, to thereby reduce the possibility that packet collision will occur in every period.